Pro-democracy demonstrators holed up in a Hong Kong university campus set the main entrance ablaze Monday to prevent surrounding police moving in, after officers warned they may use live rounds if confronted by deadly weapons.

Barricades burned in the main road next to Hong Kong Polytechnic University on Saturday night Barricades burned in the main road next to Hong Kong Polytechnic University on Saturday night Photo: AFP / Ye Aung Thu

The police warning, which came after one officer was struck by an arrow, marked a further escalation of the near six-month crisis engulfing the city.

China has repeatedly warned that it will not tolerate dissent, and there are growing concerns that Beijing could intervene directly to end the spiralling unrest.

Hardline protesters in Hong Kong threw molotov cocktails shortly before 10.30 am after police fired tear gas Hardline protesters in Hong Kong threw molotov cocktails shortly before 10.30 am after police fired tear gas Photo: AFP / Philip FONG

Several loud blasts were heard around dawn on Monday before a wall of fire lit up an entrance to the Hong Kong Polytechnic University (PolyU), AFP reporters said, as what appeared to be a police attempt to enter the campus was repelled by protesters determined to hold their ground.

Hong Kong Polytechnic University in Kowloon has become a key battleground Hong Kong Polytechnic University in Kowloon has become a key battleground Photo: AFP / Philip FONG

Police said they had fired three live rounds in the early hours of Monday at a protest site near the university but that no one appeared to have been hit.

Intense clashes throughout Sunday, which saw a police officer hit in the leg by an arrow and protesters meet police tear gas with volleys of petrol bombs, rolled overnight across the Kowloon district, as a call went out to defend the besieged campus.

A protester attempts to snuff out a tear gas canister fired by police A protester attempts to snuff out a tear gas canister fired by police Photo: AFP / Philip FONG

There, protesters had hunkered down under umbrellas from occasional fire from police water cannon and hurled Molotov cocktails at an armoured vehicle, leaving it ablaze on a flyover near the campus.

Exchanges of tear gas and petrol bombs broke out as protesters occupying Hong Kong Polytechnic University in Kowloon fight to keeping their stranglehold on a major transport link Exchanges of tear gas and petrol bombs broke out as protesters occupying Hong Kong Polytechnic University in Kowloon fight to keeping their stranglehold on a major transport link Photo: AFP / Ye Aung Thu

Police declared the campus a "riot" scene -- rioting is punishable by up to 10 years in jail -- and blocked exits as spokesman Louis Lau issued a stark warning in a Facebook live broadcast.

"I hereby warn rioters not to use petrol bombs, arrows, cars or any deadly weapons to attack police officers," he said.

Exchanges of tear gas and petrol bombs broke out as protesters occupying Hong Kong Polytechnic University in Kowloon fight to keeping their stranglehold on a major transport link Exchanges of tear gas and petrol bombs broke out as protesters occupying Hong Kong Polytechnic University in Kowloon fight to keeping their stranglehold on a major transport link Photo: AFP / Ye Aung Thu

"If they continue such dangerous actions, we would have no choice but to use the minimum force necessary, including live rounds, to fire back."

Activists hurled petrol bombs at advancing water cannon Activists hurled petrol bombs at advancing water cannon Photo: AFP / Ye Aung THU

Three protesters have been shot by police in the unrelenting months of protests, but all in scuffles as chaotic street clashes played out -- and without a sweeping warning being given by a force that overwhelmingly depends on tear gas, water cannon and rubber bullets.

Fear gripped protesters still trapped inside the campus -- whose occupation is a twist in tactics by a leaderless movement so far defined by its fluid, unpredictable nature.

Police deploy water cannon against protesters occupying the campus of the Hong Kong Polytechnic University, now a key battleground as the demonstrators fight to keep a stranglehold on a major tunnel nearby, blocked since TuesdayCOMPLETES 1MC2AF Police deploy water cannon against protesters occupying the campus of the Hong Kong Polytechnic University, now a key battleground as the demonstrators fight to keep a stranglehold on a major tunnel nearby, blocked since Tuesday Photo: AFPTV / Yelim LEE

"I feel scared. There's no way out. All I can do is fight to the end," one protester joining the barricade in front of the university building said early Monday.

Owen Li, a PolyU council member and student, said panic had taken hold of the few hundred protesters believed to be holed up.

"Many friends feel helpless... we appeal to all of society to come out and help us."

Throughout Sunday, activists parried attempts by police to break through into the campus, firing rocks using a homemade catapult from the university roof, while an AFP reporter saw a team of masked archers -- several carrying sports bows -- patrolling the site.

Violence has worsened in recent days, with two men killed in separate incidents linked to the protests this month.

Chinese President Xi Jinping this week issued his most strident comments on the crisis, saying it threatened the "one country, two systems" model under which Hong Kong has been ruled since the 1997 handover from Britain.

Demonstrators last week engineered a "Blossom Everywhere" campaign of blockades and vandalism, which forced the police to draft in prison officers as reinforcements, shut down large chunks of Hong Kong's transport network and closed schools and shopping malls.

The protests started against a now-shelved bill to allow extradition to China but have billowed to encompass wider issues such as perceived police brutality and calls for fully free elections in the former British colony.

A poster circulating on social media called for the "dawn action" to continue on Monday.

"Squeeze the economy to increase pressure," it said.